Generally, if a common base station is installed in a region where the number of subscribers is relatively smaller than the number of subscribers covered by a base station, waste occurs. In this region, a small-sized base station is installed in order to prevent such waste in advance. In this case, the small-sized base station is called “a remote unit”.
A remote unit and a base station (a main unit) serving as the main are connected to each other via a costly optical cable or a coaxial cable.
Further, for network synchronization between the remote unit and the main unit, clocks such as pulse-per-second (PPS), a reference clock (10 MHz) and TOD (Time Of Day) are required.
Conventionally, there are usually two kinds of network synchronization apparatuses between the remote unit and the main unit.
For a first type network synchronization apparatus, network synchronization is implemented in such a way that the main unit generates both a reference clock and a system clock, which are required in the remote unit, and sends those clocks to the remote unit via a communication cable (an optical cable or a coaxial cable), and the remote unit receives the clocks through the communication cable.
For in a second type network synchronization apparatus, each of the main unit and the remote unit includes an expensive global positioning system (GPS) receiver, and continuously inter-compares a pulse-per-second (PPS), a reference clock (10 MHz) and TOD received from satellites using a time/frequency-generating card and corrects time generated internally and a time length, so that each of the main unit and the remote unit is synchronized to the standard time.
However, the conventional network synchronization apparatus between the remote unit and the main unit has the following problems.
That is, in case of the first type network synchronization wherein the main unit generates all the clocks needed in the remote unit and transmits them to the remote unit via the communication cable, there are lots of clocks transmitted. Thus, there is a limitation in that a single communication cable cannot be used. Also, there is a disadvantage that an expensive communication cable such as an optical cable and a coaxial cable has to be used in terms of accuracy in clock transmission.
In addition, the second type network synchronization apparatus employing the GPS receiver has a disadvantage that a costly GPS receiver is used, and hence the price of the system is increased.
More particularly, with rapid development of communication markets, if the same function is performed in the same solution under the circumstances where the system has been competitively developed, lots of limitations occurs such as the demand for development of a lower price system.